This invention relates to electrical power system protection devices and more particularly to apparatus which limits circuit current during fault conditions.
The interruption of fault currents in a high powered electrical system is a technically difficult task when fault currents can rise to hundreds of kiloamps. One common means of current interruption utilizes a massive circuit breaker to create arc voltage between separating contacts which finally exceeds the system voltage. These breakers are subject to severe breaker arc contact erosion due to long duration, high current arcing during which inductive energy of the power system is dissipated in the arc. This contact erosion by arcing can be decreased by using a secondary breaker which initially commutates the fault current into a low inductance shunt resistor thereby heating the resistor to increase its resistance and limit current. Final interruption of this reduced current is then accomplished by a primary breaker. This scheme, which is used in direct current subway and people mover systems, entails less contact damage but requires two series connected circuit breakers.
Another current interruption scheme uses the series connection of a current limiting fuse and a standard circuit breaker. In that configuration, the fuse acts to limit current and open the circuit under massive fault current conditions. For more moderate fault currents, the fuse remains intact and the circuit breaker alone performs the interruption function. Another current limiting scheme which is suitable for smaller and generally compact electrical systems involves the dropping of generator field excitation when a fault is sensed. However, instead of simply isolating the fault, the entire generating system is necessarily disabled. In addition, the response time of generator field control is slow and dangerously high fault currents may be unavoidable.